The Solar System

A montage of the planets and the Earth's Moon. These images were taken by a variety of spacecraft. Source: Courtesy NASA/JPL-Caltech

The Sun and the planets formed around 5 billion years ago, from a cloud of gas and dust left behind by dying stars. This was mostly Hydrogen and Helium, but there were traces of the heavier elements which would allow the formation of solid planets and, eventually, life.

The cloud collapsed into a rotating disk, with almost all of the material concentrated in a central bulge, where the Sun would form. At certain distances out from the centre, mass started to accumulate and gravity pulled in more and more material. The planets were beginning to form. They swept the area around them clear of small pieces of dust and rock as they orbited. They also frequently collided with larger rocks, growing in the process. Eventually they reached their current size. The material left became the comets and asteroids.

For another billion years or so stray comets and asteroids continued to bombard the planets. Many planets and moons, such as Mercury and our Moon, still show the scars from this period in the form of craters.

Today we can see the same process taking place around other stars, such as several young stars within the Orion Nebula.

The Solar System Today

The Solar System today consists of the Sun, 8 planets, at least 3 dwarf planets, at least 63 moons and countless other small bodies including asteroids, comets and Kuiper Belt objects. At the centre is the Sun, its gravity dominating the system and locking the others into their particular orbits.

The Solar System TodaySource: IAU/Martin Kornmesser.

The area around the Sun can be divided into several distinct regions:

In the inner solar system are the four terrestrial, or rocky, planets: Mercury, Venus, Earth and Mars. These are all relatively small and made mostly of rock, so they have solid surfaces.

Beyond the orbit of Mars, marking the edge of the inner solar system, is the Asteroid Belt, made up of millions of rocks, varying in size from pebbles to rocks the size of Melbourne. The largest of these, Ceres, is now considered a dwarf planet.

In the outer solar system are the four Gas Giants: Jupiter, Saturn, Uranus and Neptune. These planets are relatively large and are made mostly of gas. They have no solid surfaces, only thick atmospheres.

Last is Pluto. Until 2006 Pluto was thought of as the ninth and smallest planet but is now considered a dwarf planet. Pluto is neither rocky like the terrestrial planets nor a Gas Giant.

Beyond the orbit of Neptune is a belt of large icy objects, possibly related to Pluto, called the Kuiper Belt. This region may be the source of short-period comets. There is at least one Kuiper Belt object, 2003 UB313 (nicknamed Xena) that is larger than Pluto and is also considered to be a dwarf planet.

Far beyond the orbits of the planets, roughly one light-year from the Sun, lies an enormous sphere called the Oort cloud, where long-period comets originate.

As well as the Sun and the Moon, five of the planets (Mercury, Venus, Mars, Jupiter and Saturn) are visible to the unaided eye and have been known since ancient times. The remainder (Uranus and Neptune) and the dwarf planets have only been discovered since the invention of the telescope.

Hi Lauren, we ran this past our Planetarium experts, who have prepared the following reply:

Planets in our solar system all vary in their composition. The inner ones like Mercury, Venus, Earth and Mars have a great deal of silicon, oxygen, nitrogen, sulphur, magnesium, and iron. The outer planets Jupiter, Saturn, Uranus and Neptune have lots of hydrogen and helium, The cold worlds beyond Neptune, like Pluto , are different too. But the chemical elements in planets is only part of the story. How they have combined as chemical compounds is more important. For example, Earth has lots of oxygen and hydrogen, but together they make water. Or Neptune has carbon, nitrogen, oxygen and hydrogen in the form of methane and ammonia. See these links to follow up...

Hi Nikki - we checked with our experts at the Planetarium, and they've responded as follows:

Planets can have rings for several reasons. By meteor impact with moons of the planet that throws material around it. By material from a moon being ejected by an internal process. By material being too close to the early planet which couldn’t form into a moon. By a moon being broken up by tidal/gravity stresses. By violent collision which another moon. Some planets have experienced one or more of these events in their history giving rise to rings of different types and sizes, while other planets have not. In our solar system the ring systems are around only the gas giants which have larger gravity and many moons giving more opportunity for one or more of these to occur but the exact process is still not fully understood.

Discovery Centre
1 December, 2012 13:55

Hi Ashutosh!

When Planets align when viewed from the Earth we call it a planetary conjunction, and these are not too infrequent. On the morning of the 3rd December, just before sunrise, Mercury will be low in the sky, then Venus and higher in the sky will be Saturn. These will be visible no matter where you are on the Earth.

The planets of our solar system frequently “line up” in one way or another as they orbit the Sun at different speeds and distances from the Sun (inner planets more quickly than outer planets slower). The alignments as seen from Earth, or from an imaginary viewpoint above the solar system looking down, are interesting but have no effect on us in any observable sense. Gravity of the Sun and of the planets, and the speed of the planets around the Sun, hold the solar system together but the vast distances between the planets means this force is very weak.

I am an 12 year old who is interested in our massive Solar System. Any way to my question i have heard there is going to be a 3 planet align in Giza, Egypt at the Giza pyramids on 3rd of December, 2012. I was wondering if we would have one in Australia preferably Melbourne it would be so cool. And if not would we be able to see this once in a lifetime opportunity or do we have to go to Giza, Egypt.
Please take some time to answer i am really keen to know.
- Ashutosh

Is it true that in December 2012 our planet will align with the milky way which will cause damage to earth, darkness for 3 days and mass earthquakes, volcanoes etc. Also, scientists claiming to be ex nasa employees are stating we are in for a massive meteorite shower which will ultimately wipe out our world. Is Nasa covering up something?

You've raised one of the hottest and controversial topics in cosmology; the multi-verse, branes, the big bang, repeated or cycling of the universe etc.. etc.. The short answer is that astronomers, cosmologists, physicists, and philosophers just don’t know for certain although they are using the best mathematics, rigorous modelling and extensive measurements. As far as travelling between supposed universes is concerned, there are no answers either as we have no certainty whether “borders” exist, how objects might cross over if they do, what mechanism might be involved. It is all very much hypothetical and speculative but grounded in the strongest research possible. It really is “cutting edge” but that’s what makes science and inquiry so exciting!

Are there other Universe's or is there just the one we inhabit & if there are would it be possible for Planets or Stars that are near where the borders are to travel through the Space of 2 Universe's ? Also - if there are more Universe's then are the borders between them clearly defined & could they be penetrated by Spacecraft or Meteors/Comets ? Also could a Universe be within a BLACK HOLE ?

Is it possible that a black hole is on a collision course with our solar system, and will suck us all up without warning, because at the moment it's on it's way to us through the emptyness of space and not sucking anything in (so we can't detect it until it's here)?

Hey JB & Connor, complex question! Basically, it's always hard to predict what will happen in the future. There are so many factors and variables that influence social and technological development. These sites should give a idea of current ideas and plans in space colonisation ...

There is no credible evidence for any of the assertions made in support of unusual events taking place in December 2012.

Another common misconception is that when there is a planetary alignment, calamities occur. Planetary alignments actually occur about every 50 or 60 years so over the last 4.3 billion years since the Earth formed we are still here. The next big planetary alignment is on September 8th 2040 when most of the planets will cluster together, this should give spectacular viewing (if you are still around.!)

Hello again! You guys are really helpful at answering my Questions! Alright, so i know the Earth wont die, and an exploding star wont kill us, but i would like to know: i heard what i think is a silly rumour that sometime in the next 100 years the planets will line up and stuff up the gravity. I dont think it will happen. Who is right????

If the star was twenty light years away, we would see the explosion in twenty years time! We will see the light of the explosion but not feel the shock waves as it too far away. Light waves travel faster and further than any other form of waves. So no, you would not die.

Riley - Mercury can't 'burn to ashes' as the surface is essentially rock, and rock doesn't behave this way when it is hot - althought the surface of Mercury is made of exceptionally hot and inhospitable rock dotted with impact craters, it is essentially similar to the moon in many ways. You can learn more at http://museumvictoria.com.au/discoverycentre/infosheets/planets/mercury/

Isaac - scientifically there's no direct evidence of life inhabiting any other planets yet, but given the size of the universe and the liklihood of habitable planets, it seems like a reasonable guess that there is life elsewhere in the universe. NASA has its own research department dedicated to searching for evidence of life elsewhere in the universe, called the NASA Astrobiology Institute; you can learn about their research at http://astrobiology.nasa.gov/about-astrobiology/

Hi Connor, great question! Although it does sound a bit frightening, the earth is constantly being bombarded by meteorites - because the earth's surface is mostly sea, we don't have many of the records of these impacts, but rest assured, they happen costantly. Thankfully, these are all pretty small and pose a low threat, nothing large enough to trigger an extinction.

There is a research unit of NASA that examines 'Near Earth Objects', asteroids,comets and meteorites that have potential to strike the earth. The website is at http://neo.jpl.nasa.gov/; we mention this to assure you that there are many experts monitoring this, and that doesn't appear to be a significant threat from an extinction caused by a large meteorite strike any time soon.

Hi Guys. I'm 13 years old and I absolutely love space! My question is: I don't believe in the "2012" idea that the world will end. But is there any possible threat of human extinction? Please answer my question!

The Moon appears inverted, or upside down, when viewed from one hemisphere compared to the other. It’s the result of the different angle viewers have from locations south or north or the equator. The effect is to make the sunlit portion appear to increase from the left side of the Moon during the month as seen from Australia (southern hemisphere) while from the right side as seen from America (northern hemisphere).

Hi Robert, Eris is a dwarf planet, with moon Dysnomia, identified in 2005 and is a significant object in its own right although very far from the Sun in a highly inclined eliptical orbit. Its size is still under question and could be a little smaller or bigger than Pluto. It is one of several TNO’s (Trans-Neptunian Objects, beyond Neptune’s orbit) discovered in recent years. Some people may confuse it with the fictional Nibiru, or ideas of “planet X” or a so-called “tenth planet”.

iv just been on youtube and descover that the planet-x or nibiru is not the proper name is name is ERIS UB313 .it is the tenth planet distence is 96.6 au three times the size of pluto this could be the one everyone is talking about 2012

Hi,
I have worked as a planetarium presenter and scientific exhibitions guide in France for the last 4 years and I would love to find a similar position in Australia. Would you be interested in a person with an academic background in astronomy to present planetarium shows or exhibitions about sciences? Do you have any link accorded to that kind of jobs as it seems to be very hard to find it in Australia?
Thank you for your time.

people are worry about 2012 the end.i have been on youtube and discoverd a lot of information on a cerlistreal body hedindg our way 2012.people are talking about today nasa has located this devestated planet on corse and heading our way .it is proff that there is an unexspeted visitor coming too town 2012 so thay say.the last visit was 4.7 billions years ago

Hi Jessica, sorry for the delay. With the new definition for “planet” a body would need to; (1) orbit the Sun (not another body), (2) be massive enough to have formed into a spherical shape, and (3) to have cleared its orbit of other objects. Pluto satisfies the first two requirements but not the third as there are other objects sharing its orbit around the Sun. However, Pluto has the honour to be one of the first in the new class of dwarf planets which are all significant objects telling us much about the early formation of the solar system. The discovery of so many bodies similar to Pluto has opened up a new chapter in our understanding our the solar system.

We have two separate factors in this excellent question but they are not connected. Jupiter’s spin or speed of rotation is simply how fast it turns on its axis (its “day”), and orbital speed is how long it takes to travel around the Sun (its “year”).

Gracie, as indicated above the Solar System developed from elements that later made life possible. But there of course may have been forms of life in other solar systems. You can read more about this on the website for NASA’s Exobiology and Evolutionary Biology Program.

Lollypop, you can find out how to research your question by reading our reply to Maddie on 20 November.

Hi Abraham, in answer to your question from a few weeks ago, good views of the Milky Way can be seen every night of the year at our latitude although it’s always best to be well away from city lights in a dark location to see it well. Overall the best time in the year would be midwinter at around midnight as by then it stretches high overhead from north east to south west horizons showing us the most expanse of the galaxy. Winter is also better than other seasons as the air is usually clearer and more stable when it’s a clear night all of which improves observing whether by naked eye, binoculars or telescope.

Hi Maddie, your first & second questions can be answered by reading the first paragraph of the infosheet above. For your third question, the sun will keep getting hotter and expand, eventually swallowing the inner planets. A great explanation can be found on the ABC's Meet the Neighbours site. There are probably no more planets in our solar system to discover but out in the universe there are billions, which we call exoplanets (extrasolar planets). As for information about the rotation and orbit of all the planets, the answers can be found under each planet's link on the NASA Worldbook site. We hope this helps!

i have four questions i would like to know. the first one is how did the solar system begin? the second one is what was before the solar system? the third one is will the solar system still exist in bilions of years? and the last one is are there other planets that we dont know about. it would be really helpful if you could answer these questions. Thanxs

Hi Clyde Primary School, thanks for all of your questions! If you take a look around the Museum Victoria Discovery Centre page, under the Astronomy link, you will find the answers to many of your questions. Maybe also look at the NASA website, http://www.nasa.gov/ and try searching particular phrases or keywords on the internet.

I know a rhyme to say if you can't think of the planets in our solar system:
My Very Energetic Mother Just Sold Us Nine Planets
look at the 1st letters of every word and you will see what i mean..... now you can't forget the planets!!

HI i've been to the planeterium before as a school group and it was amazing. I just question the planets that are out there are the really important and in my meanings i mean why? As we know so far no-one lives on those planets and why is Earth the only one with us but the 8 planets left are only facts and things we study on in the sky....so what really are we focusing on these planets? THANK YOU Jane

Hi Shashank, NASA commenced a mission to Pluto in 2006. The unmanned spacecraft, named New Horizons, will reach its closest point to Pluto in 2015.The spacecraft is fitted with scientific instruments which will enable NASA to study the atmospheres, surfaces, interiors and environments of Pluto and its distant neighbours. You can read more about this mission, and others here.

Hi Mark, you're probably seeing Venus, which is slightly brighter than Jupiter at the moment, even though Jupiter is as close to Earth as it ever gets (it won't be this close again until 2022). Venus is in the western sky and Jupiter is in the east. Saturn cannot be seen at the moment as it is too close to the sun. Hope this helps.

Why can't all planetaria like yours update old NASA data to include in text if not slides the dwarf planets (Charon,Eris,Makemake,Haumea,Orcus,Quaoar ) and their moons (like Weywot, Nix,Hydra, Vanth, Hi'iaka, Namaku and others) that have been named from the Edgeworth-Kuiper disc and the Opik-Oort cloud's Sedna? on display and lecture material?

Jupiter is visible from both the city and the country, although there is much better seeing conditions in the country due to there being less light pollution. Check the Skynotes September 2010 for instructions about where to find Jupiter in the night sky this month.

Hi Peter. Who knows what future technologies will allow in regards to space travel but, at the moment, travel to extra-solar objects such as other galaxies and planets does look a little unlikely! As to the expanding universe and reactions, there is no definitive answer to the expanding and accelerating Universe at this time but some recent hypotheses include theories involving Dark Matter or Dark Energy. Take a look at the information on the Museum website regarding the ‘Big Bang’ and also NASA’s website regarding the ‘Big Bang’ and Dark Energy.

If Einstein said the max speed is the speed of light how can we ever get to other stars/planets in our galaxy?- yet alone to galaxys seen by Hubble.I know we are 1000 light yrs across our Milky way.If the universe is expanding like someone throwing out a handfull of sand , how do we not have any reaction.Thanks Peter

I have received an email, including a powerpoint presentation, claiming that Mars is approaching its closest point to the earth in the last 5000 years. It is claimed that Mars will be about the same size as the moon, and will rise at 10pm on 1 aug, azumeth at 3am.
Is this correct, even for USA or UK, (times are wrong for here).

Hi Marleen, the Planetarium staff believe the star referred to is Canopus, the second brightest star in the night sky. Sirius, the brightest star in the night sky rises just before dawn, but unlikely to be seen, being too low to the horizon. The brightest object to be seen in the morning night sky is in fact Jupiter. This can still be seen high in the northern sky even as the sky lightens and turns blue.

Hi Pasquale, the planets do indeed rotate around the sun as it says in the article above. The apparent 'movement' of the sun across the sky is actually caused by the rotation of the earth - one rotation equals one earth day. You can find further information and links on our website here and here.

Hi Alan - our Planetarium staff think that the bright star you are seeing is the second brightest star in the night sky - Canopus. It is low in the south west in the early morning, just before sunrise.

As can be seen from our Skynotes, http://museumvictoria.com.au/planetarium/discoverycentre/skynotes/skynotes-december-2009/ Mercury can be seen in the evening sky just after the Sun sets in the west. Venus is much too close to the Sun at the moment to really see it, but you may just be able to pick it out in the morning in the east just before sunrise.

Is Venus and mercury in the Eastern Sky as the Sun is Setting in the West at around 8.40pm in the Evening?
Can you tell me where to observe Venus and Mercury in the night Sky just after the Sun has Set. Are they to be viewed in the Western Sky or in the eastern Sky at this Hour?

The declassification of Pluto apparently produced heated debate among astrologers particularly on blogs. It seems that just because it's not a planet anymore doesn't mean that it is not considered significant by the astrological community. Just like for astronomers, Pluto's re/declassification does not detract from its interest and, possibly, only increases that interest.

Email(will not be published)Please enter an email addressPlease enter a valid email

CommentPlease enter a comment

How many letters are in the name Victoria?Please answer the anti-spam questionAnti-spam question incorrect

We love receiving comments, but can’t always respond.

Discovery Centre
15 September, 2009 10:22

Hi Milka! It's great that you've got such an interest in astronomy. We'd suggest that you have a look at the other infosheets available on this site (lots of your questions are answered there), and if you'd like to learn more, have a look at Australian Astronomy's "Learning More" page for some fun and useful links!

Milka 7 years old have some quations.
I want to know what the other galaxys
are called? I want to know why Jupiter's red spot is 3 times bigger then Earth? I want to know why our galaxy is called The milky way? I want to know why thay made Pluto out of the gang and made Pluto a dwarf planet?

Space is a pretty big topic but this website is a good place to learn about it. We have lots of great information including sky maps, moon phases, Skynotes and infosheets. You might also want to visit the Planetarium to see one of our shows. The NASA website also has a vast amount of information on space and should keep you busy for ages!

Hi I have a multiple choice question which i really cannot answer and am wondering if you could help! It says, Jupiter has:
A. 12 rings (as far as im aware it only has four???)
B. 23 satellites (as far as I'm aware it has 63???)
C. 19 moons (I though it was 63 as above)
D. A star of its own (Since the above answers seem wrong to me Im thinking that this may be the answer but I do not understand how. Could it be referring to the sun?)
If you do have any information which could help me answer this it would be a great help! Thanks alot!

A "light year" is a measurement of distance that corresponds to the distance travelled by light in a year, so if a star is 321 light years from the sun, then that star's light will take 321 years to reach the sun.

Hi Natali, The term "Dwarf Planet" is a definition created by the International Astronomical Union. From their website: A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. There are currently five recognised dwarf planets, as listed here.